The dominant trait of the fish color is orange, the recessive trait of the fish color is gray. The gray and orange fish reproduce and overproduce since they know that some may die.
There is a variation in the fish color. The offspring are mostly orange since that is the dominant trait. There aren't many gray fish since that is the recessive trait.
A shark watches the fish from far away ready to attack. They only see the orange fish since they don't blend in with the rocks. This makes them purposely hunt down the orange fish.
The gray fish thrive
Nature selected against the orange fish. The shark only saw the orange fish so most of them died and the gray fish stayed alive. The fish develop an adaptation caused by most of the orange fish to die.
Now that most of the orange fish died, most of the fish have the recessive gray color trait but since orange is the dominant trait, the orange fish population will never go away.
The gray fish thrive since they can camouflage and hide from predators easier. This leaves the gray fish to reproduce and now there are more gray fish offspring.